Lightguide fiber is generally formed by locally and symmetrically heating a cylindrical silica glass rod which is called a preform. Typically, the preform is 7 to 25 mm in diameter and 100 cm in length and is heated to temperatures in excess of 2000.degree. C. As the preform is fed into a hot zone, fiber is drawn from the molten material, yielding a substantial replica of the preform cross-section.
Due to the temperatures involved and to avoid potential damage to the fiber surface, fiber cannot be drawn through a die. Consequently, the surface of the molten material is a free boundary whose shape is determined by a balance between viscous forces of the glass material, surface tension and shear forces.
When the glass is in a molten state, it is susceptible to mechanical, accoustical and thermally-induced disturbances and also to variations in diameter which occur while the process approaches its equilibrium state. An additional source of variation which is of a slowly varying nature results from changes in the preform diameter.
In a draw system which is well-known in the industry, the preform is fed into a heating zone where it is necked-down in a melt cone to the fiber size to permit fiber to be pulled from it. The diameter is measured at a point shortly after the fiber is formed, and its measured value is provided as an input to a control system. Within the control system, the measured fiber diameter is compared to a desired value and an output signal is generated to adjust the draw speed, if necessary, to correct the diameter. After the fiber diameter is measured, a protective coating is applied and is cured. Afterwards, the coated fiber is spooled for testing and storage prior to subsequent cabling operations. See the Western Electric Engineer, Winter 1980 issue, article beginning on page 49.
During the fiber drawing process, particles of dust and dirt can pass from the heating zone to the glass preform into the fiber surface. As a result, the tensile strength of the fiber is reduced considerably and attenuation is increased. A discussion of these problems is contained in an article by H. Aulich et al "Preparation of Optical Fibers of High Tensile Strength", Siemens Forschungs-Und Entwicklungsperreicht, Volume 7, No. 3, 1978 pps. 165-168.
Four types of heat sources have been used in an apparatus for drawing lightguide fiber, the simplest of which is an oxygen-hydrogen burner which is called a torch. In one configuration, a plurality of torches are directed toward the preform. The torch approach is clean in that there are no contaminants in the vicinity of the molten glass, but variations in the fiber diameter may occur due to the turbulence of the torch flame and the open environment surrounding the necked portion. In use of the torch, the primary mode of heating is by conduction from the flame to the glass. Because of the non-contaminating character of the torches, the fibers produced have been, in general, stronger than those produced with electrical furnaces. In addition, the string up in these kinds of heat sources is relatively simple to accomplish and they do not require purge gasses nor the consumption of energy during periods of non-use. Also, the melt cone of the preform is easy to observe and to control.
Diameter control when using a torch to heat a preform has been a problem also because of the manner in which the flames have been applied. The flames of the torch have temperature regions or zones which vary within relatively short distances. In torch heating, fuel and oxidizing gases emerge from supply ducts to create a flame having three somewhat distinct zones. First, in order from a nozzle of the torch, there is the mixing or precombustion zone, followed by a combustion zone which is the hottest part of the flame. The last or outer zone in which the combustible gases mix with outside air is called a plume and is a zone of unsteady temperatures. It has been typical in the prior art for the torch to be positioned so that the target surface which is to be heated lies generally in the plume of the torch flame. This contributes to the nonuniformity of the temperature. Multi-nozzle torches called ring burners in which the nozzles are directed radially have been tried, but have not produced uniform temperature fields because the gases emerging from one nozzle interact with with those of the radially opposed nozzle thereby causing variability.
Some routineers have used an arrangement of only two nozzles, but then, in order to distribute the heat uniformly about the preform, the preform must be rotated. Since preforms are not perfectly straight, the rotation causes the preform to wander into and out of different temperature fields causing diameter fluctuation.
There is also another problem with respect to the ring type burner. Generally, as soon as the melt cone begins to form, the distances from the torches of the ring burner to the preform are somewhat distorted from those distances at the outset. Undesirably, once the distances are set and those distances are distorted because of the melt cone, design parameters can become changed.
A second heat source which is also very clean is a laser. By the use of a rotating lens or scanning galvanometers, the laser's energy is distributed uniformly about the preform. The energy is absorbed by the surface of the preform and the interior is heated by conduction. The laser is a clean energy source since the environment surrounding the molten glass is independent of the laser. Although diameter variation is much lower than that achieved with torches, the molten glass is subject to convection disturbances. While the laser has proven to be a useful laboratory tool, other sources offer nearly the same cleanliness, better environmental control, and much lower investment and operating costs.
The other two sources which have been used are furnaces, which differ significantly in interval construction. A graphite furnace uses a graphite ring which is heated resistively or inductively to heat the preform by radiation. At elevated temperatures, though, graphite reacts readily with oxygen, and must be surrounded by flowing a protective gas such as argon or nitrogen into the furnace. The flow of gas must be carefully controlled to prevent disturbances to the necked-down molten glass region. In addition, due to the high operating temperature of the furnace elements, there is a risk of contamination of the preform and consequently, reduced fiber strength.
An alternate furnace design uses zirconia rings, which are RF inductively heated, to heat the preform by convection and radiation. This furnace configuration has the advantage that zirconia does not require a protective inert atmosphere, and consequently the preform may be drawn in a relatively quiescent environment without the expense of a protective gas.
Problems have been encountered with the zirconia furnace. It has been found that insulation material therein generates particulate matter. Should this matter contact a fiber which is being drawn, it causes flaws and weakens the fiber. Also, it has been found that a substantial percent of fiber breaks during proof testing are caused by particulate matter induced flaws. It should be apparent that a desirable heat source is one which does not contaminate the glass.
Cost is also another factor that makes the zirconia furnace not altogether desirable. Once it is brought up to an operating temperature above 1900.degree. C., the muffle tube thereof which is formed by zirconia rings called elements cracks if it is cooled down. As a result of its sensitivity to thermal shock, the zirconia furnace must remain at an elevated temperature thereby increasing energy consumption. Moreover, if the preform were to touch a zirconia element, it would become adhered thereto, terminating the life of the element.
From the foregoing discussion, it should be apparent that a torch arrangement offers inumerable advantages over the other identified heat sources. A torch arrangement provides a much cleaner environment of heat than a furnace. The water vapor product of combustion does not condense at the high temperatures. As a result, particulate matter is not deposited in the surface of the preform and the product fiber has a higher strength. However, what is needed and what the prior art seemingly does not provide is a torch arrangement with all its attendant advantages and yet one which allows suitable control of the diameter of the drawn lightguide fiber.